The present invention relates to compounds and compositions for use as liquid crystal phase in electro-optic display devices and, specifically, to certain novel alkylsubstituted N-benzylideneanilines as well as to liquid crystal compositions containing such novel compounds, and to improved methods of producing alkylsubstituted N-benzylideneanilines.
More than 7000 compounds are known to be capable of existing in a liquid crystalline state, i.e. exhibiting one or more mesomorphic states or mesophases over a temperature range near, or adjacent to, their melting point temperature. In their liquid crystalline mesophase such compounds--generally referred to as "liquid crystals" or "LC" for short--behave neither as liquids proper nor as crystalline solids because while behaving as liquids they show a molecular alignment or orientation, e.g. as evidenced by X-ray diffraction, of the type that is characteristic of crystalline solids. Texture observations of various liquid crystals under a polarizing microscope indicate the existence of different types of mesophases, i.e. the so-called nematic and smectic mesophase, and sub-types thereof are known and are designated by capital letters, for example as smectic phases B, E and G.
As is commonly known, such liquid crystals are used for electro-optic display devices--commonly called "LC-displays"--in order to distinguish them from other types of electro-optic devices, for example those employing gas-discharge devices or semiconducting solids including light-emitting diode displays and the like. The term "display" is used to refer to the capacity of such devices to show or display predetermined symbols including alphanumerical signs. In general, the electro-optic display device of the type operating with liquid crystals comprises a cell including electrodes, a liquid crystal mass or composition as a dielectric phase or component between the electrodes and electronic means to selectively activate specific electrode areas. Such LC-displays are well known in the art and are produced commercially in increasing amounts for such purposes as digital wrist watches and the like.
For commercial production of LC-displays only a limited number of organic mesophase or LC compounds can be used. This is due to the fact that most liquid crystal compounds are solids at room temperature and/or exhibit a very narrow operational temperature range. One of the few liquid crystal compounds known in the art has been described by G. Gray, M. Harrison and I. Nash in Electronic Letters, Vol. 9, No. 6, page 130, 1973 and is 4-cyano-4'-n-hexylbiphenyl melting at 13.5.degree. C. Virtually, any LC-display for operation at normal ambient temperatures, i.e. the general range of from about -20.degree. C. to about +70.degree. C. and including the room temperature range of from about 15.degree. C. to about 25.degree. C., includes a composite mixture of LC-compounds that are solid at room temperature either as a mixture of one or more LC-compounds or, preferably, as a mixture of one or more normally solid LC-compounds with one or more other compounds that are not liquid crystals proper but have a chemical structure similar to that of the LC-compounds and thus are capable of forming eutectic multicomponent mixtures with LC-compounds. Most LC-displays for commercial purposes and notably the so-called "twisted nematic displays" (cf. Appl. Phys., 1975, pages 101-106) employing effects discovered by well known authors, such as J. H. Heilmeier, L. A. Zanoni, M. Schadt and W. Helfrich (cf. Appl. Phys. Letters No. 13, 1968, page 91; and Appl. Phys. Letters No. 18, 1971, page 127) use LC-compositions in the nematic mesophase and the art is replete with specific nematic multicomponent liquid crystal compositions, German Patent application DT-OS 2,344,732 being recited here as a representative example only.
Research efforts with the aim to find additional low-melting LC-compounds that have a relatively simple chemical structure, and to develop improved LC-compositions are being continued, however, for the following reasons: various and, to some extent, contradictory requirements have to be met with regard to clearing point temperature, nominal melting point temperature, operative temperature range, operating voltage, chemical stability of components and physical stability of composition so that more than three different constituents will have to be used for an LC-composition that meets such requirements. Accordingly, the resulting composition is a fairly complex mixture and this complexity tends to increase the danger of chromatography-type separation phenomena upon introduction of the LC-composition into an LC-display cell. With such complex mixtures, the desired eutectic composition is not well defined and can be fully achieved in exceptional cases only. Maintaining a eutectic state, however, is essential when using such ternary, quaternary or even more complex compositions as this tends to be of critical importance for a reliable continuous operation of LC-displays. When using prior art non-eutectic compositions, an intermediate cooling of the cell below the lower limit of the operable temperature range results in a separation that tends to be practically irreversible so that the LC-display will cease to be operative. Specifically, when a component of the composition crystallizes upon cooling in the sense of becoming solid, the properties of the composition will be changed in a virtually irreversible manner as regeneration of the composition upon subsequent warming-up of the LC-display to a temperature within the normally operative temperature of the cell cannot be expected because of the minute dimensions of the cell space that includes the LC-composition so that a return into the original state is precluded for all practical purposes.
Accordingly, there is a continued substantial demand for new specific LC-compounds suitable per se or as a constituent of binary and not more than ternary composite LC-mixtures or compositions that have improved characteristics for use in standard LC-display devices.
The use of the broad general class of compounds of formula ##STR2## wherein R.sup.1 and R.sup.2 are selected from the group of alkyl, alkoxy and alkylcarboxy groups comprising of from 1 to 7 C-atoms as an additive for specific LC-compositions has been suggested in some publications, e.g. German Patent Specification DT-OS 2,316,864, without, however, reciting a single specific compound in which both R.sup.1 and R.sup.2 are alkyl groups.
All specific known compounds of this type where both R.sup.1 and R.sup.2 are alkyl groups (methyl/methyl and ethyl/methyl) are known to be isotropic, i.e. having no mesophase as explained above, and have high melting points. Accordingly, such compounds are not suitable at all for LC-compositions. One other specific known compound (formula (V) wherein R.sup.1 is the methyl group and R.sup.2 is the n-butyl group) is isotropic as well but has been suggested for use as an additive in LC-compositions to lower the melting point of the entire mixture (cf. German Patent applications DT-OS 2,214,967 and DT-OS 2,307,148).
Summarizing the art it can be said that p,p'-dialkylbenzylideneanilines as a class did not seem to offer particular advantages for the art of LC-display devices.